Mulberry Grove Flood Mitigation, Great Barrier Island

Stormwater analysis for catchment area

Bartley Consultants were commissioned by the former Auckland City Council to investigate flooding occurring at Mulberry Grove School, Great Barrier Island. The flooding occurred whenever the adjacent stream exceeded the capacity of the road culverts and the road was overtopped. The flooding extended into the carpark area of the adjacent school.

Bartley Consultants carried out a stormwater analysis for the catchment area upstream of the road culverts in accordance with the former ARC document TP108. The US Army of Corps Hydraulic Engineering Centre (HEC) Hydraulic Modelling System (HMS) and River Analysis System (RAS) software was used to model the existing stormwater flows, determine the capacity of the existing culverts and to model potential flood mitigation options.

A number of flood mitigation options were assessed and in conjunction with Auckland City Council a decision was made to install a new 2.5m wide by 1.5m high box culvert next to the existing box culverts. Bartley Consultants prepared the supporting design documentation required for the resource and building consent applications. Construction drawings were also prepared and the implementation of the works was supervised by one of Bartley Consultants experienced site engineers. One of the main challenges on this project involved keeping the road open during the construction of the culvert. This required a collaborative approach, between our site engineer and the road maintenance contractor on Great Barrier Island, in order to find workable solutions using locally available resources. The work was completed in 2010.